From b11429d77e448c941bbab2a364a443400e4d7a22 Mon Sep 17 00:00:00 2001
From: Anastasios Papagiannis <tasos.papagiannnis@gmail.com>
Date: Thu, 1 Feb 2024 10:40:55 +0000
Subject: [PATCH] Compute a stable start_time

We first calculate the boot time of the Linux. Then we add to that
whatever we get from the kernel space.

Signed-off-by: Anastasios Papagiannis <tasos.papagiannnis@gmail.com>
---
 cmd/tetragon/main.go   |  3 +++
 pkg/ktime/ktime.go     | 37 +++++++++++++++++++++++++++++++++++++
 pkg/process/process.go |  2 +-
 3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/cmd/tetragon/main.go b/cmd/tetragon/main.go
index dda58023f8c..2719ccdc0c8 100644
--- a/cmd/tetragon/main.go
+++ b/cmd/tetragon/main.go
@@ -35,6 +35,7 @@ import (
 	"github.com/cilium/tetragon/pkg/fileutils"
 	"github.com/cilium/tetragon/pkg/filters"
 	tetragonGrpc "github.com/cilium/tetragon/pkg/grpc"
+	"github.com/cilium/tetragon/pkg/ktime"
 	"github.com/cilium/tetragon/pkg/logger"
 	"github.com/cilium/tetragon/pkg/metrics"
 	"github.com/cilium/tetragon/pkg/metrics/metricsconfig"
@@ -147,6 +148,8 @@ func tetragonExecute() error {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
+	ktime.InitializeBootTime()
+
 	sigs := make(chan os.Signal, 1)
 	signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM, tgsyscall.SIGRTMIN_20,
 		tgsyscall.SIGRTMIN_21, tgsyscall.SIGRTMIN_22)
diff --git a/pkg/ktime/ktime.go b/pkg/ktime/ktime.go
index 84f8de760ca..22ddd4205b8 100644
--- a/pkg/ktime/ktime.go
+++ b/pkg/ktime/ktime.go
@@ -4,6 +4,7 @@
 package ktime
 
 import (
+	"sort"
 	"time"
 
 	"github.com/sirupsen/logrus"
@@ -11,6 +12,32 @@ import (
 	"google.golang.org/protobuf/types/known/timestamppb"
 )
 
+var (
+	bootTime int64
+)
+
+func InitializeBootTime() {
+	var times []int64
+
+	// Try to get the linux boot time in a microsecond resolution.
+	// As there are 2 calls to get the time "time.Now()" and "unix.ClockGettime"
+	// we have a race. So we run this 1024 times and we keep the lowest value.
+	// We expect after tetragon restart to always get the same value.
+	for i := 0; i < 1024; i++ {
+		now := time.Now()
+		currentTime := unix.Timespec{}
+		if err := unix.ClockGettime(int32(unix.CLOCK_BOOTTIME), &currentTime); err != nil {
+			continue
+		}
+
+		t := time.Unix(currentTime.Sec, currentTime.Nsec).UnixMicro()
+		b := now.UnixMicro()
+		times = append(times, b-t)
+	}
+	sort.Slice(times, func(i, j int) bool { return times[i] < times[j] })
+	bootTime = times[0]
+}
+
 func ToProto(ktime uint64) *timestamppb.Timestamp {
 	return ToProtoOpt(ktime, true)
 }
@@ -37,6 +64,7 @@ func NanoTimeSince(ktime int64) (time.Duration, error) {
 	diff := currentTime.Nano() - ktime
 	return time.Duration(diff), nil
 }
+
 func DecodeKtime(ktime int64, monotonic bool) (time.Time, error) {
 	var clk int32
 	if monotonic {
@@ -52,3 +80,12 @@ func DecodeKtime(ktime int64, monotonic bool) (time.Time, error) {
 	t := time.Now().Add(time.Duration(diff))
 	return t, nil
 }
+
+func DecodeKtimeStable(ktime int64) time.Time {
+	return time.UnixMicro(bootTime + ktime)
+}
+
+func ToProtoOptStable(ktime uint64) *timestamppb.Timestamp {
+	decodedTime := DecodeKtimeStable(int64(ktime) / 1000) // nano to micro
+	return timestamppb.New(decodedTime)
+}
diff --git a/pkg/process/process.go b/pkg/process/process.go
index 2f31901ecde..d6dd23eec54 100644
--- a/pkg/process/process.go
+++ b/pkg/process/process.go
@@ -358,7 +358,7 @@ func initProcessInternalExec(
 			Binary:       binary,
 			Arguments:    args,
 			Flags:        strings.Join(exec.DecodeCommonFlags(process.Flags), " "),
-			StartTime:    ktime.ToProtoOpt(process.Ktime, (process.Flags&api.EventProcFS) == 0),
+			StartTime:    ktime.ToProtoOptStable(process.Ktime),
 			Auid:         &wrapperspb.UInt32Value{Value: process.AUID},
 			Pod:          protoPod,
 			ExecId:       execID,